1. Field of the Invention
The invention relates in general to protective relay systems for protecting high-voltage transmission line sections in a three-phase electrical power system.
2. Description of the Prior Art
U.S. Pat. No. 4,275,429, which is assigned to the same assignee as the present application, discloses a new and improved protective relay system which provides the functions of an electromechanical pilot-wire relay, without the necessity of having a continuous, metallic conductor connected between the points to be compared. This relay may use many different types of communication links, such as an optical link, microwave, power line carrier, or telephone channels. The telephone channel, for example, may be the dedicated, uniterruptible Bell Telephone System 3002 channel.
Copending application Ser. No. 339,031, filed Jan. 13, 1982, now U.S. Pat. No. 4,408,246, sets forth another embodiment of an evaluation function which may be used in the protective relay system disclosed in the hereinbefore mentioned U.S. patent. Copending application Ser. No. 239,917, filed Mar. 3, 1981, now U.S. Pat. No. 4,380,746 discloses a new and improved pulse modulator, which may be used in the modem of the relay of the hereinbefore mentioned U.S. patent.
Concurrently filed application Ser. No. 397,944, filed July 13, 1982, entitled "Demodulator", discloses a new and improved pulse demodulator which may be used in the modem of the relay system of the hereinbefore mentioned U.S. patent.
Direct transfer trip (DTT) is an important function in the application of protective relays, as it provides the capability of tripping a circuit breaker at a remote location, without supervision by fault detectors. It would be desirable to be able to add a DTT function to the protective relay system of U.S. Pat. No. 4,275,429 and application Ser. No. 339,031. To add the DTT function via a dedicated communication channel would be a simple task, but not economically attractive. From the economic viewpoint, it would be desirable to be able to utilize the same channel used by the protective relay. However, conventional techniques would greatly complicate the design. Thus, it would be desirable to add a DTT function to this protective relay system, using the communication channel of the protective relay, without resorting to an involved, complicated design, which would not only add cost, but it could adversely affect the dependability, reliability, and security of the relay system.
In a preferred embodiment of this pilot relay, pulse period modulation (PPM) is used, with preferred embodiments of the pulse modulator and pulse demodulator being set forth in the hereinbefore mentioned commonly assigned applications. This modem is suitable for any type of communication channel, including the hereinbefore mentioned 3002 telephone channel, even though the latter has a very limited usable bandwidth. For this application, the modem carrier frequency is chosen at 1.7 KHz with a maximum deviation of about .+-.200 Hz. There is minimum attenuation and envelope delay in this narrow frequency band. Thus, it would be desirable to add a DTT function to this protective relay system while using the modem described in the hereinbefore mentioned patent applications, if this can be accomplished without complicating or expanding the basic modem circuits.